Hair dyes can be classified by the dye to be used or according to whether they have bleaching action on melanin or not. Typical examples include a two-part permanent hair dye composed of a first part containing an alkali agent, an oxidation dye and optionally, a direct dye such as nitro dye and a second part containing an oxidizing agent; and one-part semi-permanent hair dye containing an organic acid or an alkali agent, and at least one direct dye such as acid dye, basic dye or nitro dye.
The two part permanent hair dye however has a drawback that color tone imparted by an oxidation dye is not so vivid. Another drawback is that the color of the hair dyed with a vivid-color producing nitro dye ordinarily employed as a direct dye markedly fades over time and becomes dull soon even if the color tone just after dyeing is very vivid (for example, refer to Patent Document 1). In order to produce a vivid color, permanent hair dyes therefore contain various direct dyes such as cationic direct dyes and nitro dyes in combination.
Direct dyes available at present however do not bring about sufficient effects. The kind of direct dyes to be used in combination with an oxidation dye is limited because they are required to have stability against alkaline peroxides during the hair dyeing process. In any case, fading progresses very rapidly owing to the loss of a direct dye caused by washing or exposure to light and this phenomenon is notable in damaged hair or porous hair (hair having pores therein).
The present inventors therefore proposed an azo dye having a dissociative proton as a useful direct dye capable of overcoming the above-described problem (for example, refer to Patent Documents 2 and 3). This direct dye however needs to be improved to have excellent resistance to light, washing, sweat, friction and heat, and needs to be stable against an alkalizing agent and oxidizing agent.    [Patent Document 1] JP-A-6-271435    [Patent Document 2] JP-A-2003-342139    [Patent Document 3] JP-A-2004-107343